Architectural precast panels are widely used in the commercial construction industry. They provide a low cost and efficient exterior paneling system for multistory buildings. Architectural panels also have the schedule advantage of being fabricated off-site and then transported to the building site when needed. Architectural precast panels are easy to install and are relatively easy to repair when compared to other forms of exterior panel construction.
Architectural precast construction relies on mechanical connectors at discrete locations that are subjected to very large forces in a blast event, posing specific design problems to the engineer. These problems can be overcome with proper detailing.
Architectural panels typically have a row of connections at the top of the panel and a second row of connections at the bottom of the panel. Some architectural panels also have a row of connections along the sides of the panels. These connections are then attached to the structure through mounting brackets that are welded to the structural steel frame or embedded in the structural concrete.
For aesthetic reasons, it is usually desired to have the panels as close together as possible. The gaps between the panels are typically filled with an elastomeric sealant. Large gaps between panels are visually unattractive and the sealant must be maintained more frequently than the architectural panels.
Multistory buildings are flexible structures that are designed to accommodate external forces. Common forces include horizontal and vertical ground forces (e.g. earthquakes) or horizontal forces (e.g. wind pressure and blast pressure).
Although the internal steel structure is flexible, the exterior architectural panels are relatively rigid in comparison. When an external force causes the building to flex the panel connections must accommodate relative movements between the flexing structure and the rigid panels. The capacity of a panel to deform significantly and absorb energy is dependent on the ability of its connections to maintain integrity throughout the blast response. If connections become unstable at large displacements, failure can occur. The overall resistance of the panel assembly will be reduced, thereby increasing deflections or otherwise impairing panel performance.
It is also important that connections for blast loaded members have sufficient rotational capacity. A connection may have sufficient strength to resist the applied load; however, when significant deformation of the member occurs the rotational capacity may be reduced due to buckling of stiffeners, flanges, or changes in nominal connection geometry, etc.
Both bolted and welded connections can perform well in a blast environment, if they can develop strength at least equal to that of the connected elements or at least to that of the weakest of the connected elements.
For a panel to absorb blast energy and provide ductility while being structurally efficient, it must develop its full plastic flexural capacity which assumes the development of a collapse mechanism. The failure mode should be yielding of the steel and not splitting, spilling or pulling out of the concrete. This requires that connections are designed for at least 20% in excess of the member's bending capacity. Also, the shear capacity of the connections should be at least 20% greater than the member's shear capacity, and steel-to-steel connections should be designed such that the weld is never the weak link in the connection. Coordination with interior finishes needs to be considered due to the larger connection hardware required to resist the increased forces generated from the blast energy.
Where possible, connection details should provide for redundant load paths, since connections designed for blasts may be stressed to near their ultimate capacity. The possibility of single connection failures must be considered, as well. Consideration should be given to the number of components in the load path and the consequences of a failure of any one of them. The key concept in the development of these details is to trace the load or reaction through the connection. This is much more critical in blast design than in conventionally loaded structures. Connections to the structure should have as direct a load transmission path as practical, using as few connecting pieces as possible.
Rebound forces or load reversal can be quite high. These forces are a function of the mass and stiffness of the member as well as the ratio of blast load to peak resistance. A connection that provides adequate support during a positive phase load could allow a member to become dislodged during rebound. Therefore, connections should be checked for rebound loads. It is conservative to use the same load in rebound as for the inward pressure. More accurate values may be obtained through dynamic analysis and military handbooks.
The protection of multistory buildings to damage from earthquakes is described in the prior art. U.S. Pat. No. 3,638,377 issued on Dec. 3, 1969 to Caspe, describes an earthquake resistant multi-story structure that isolates the structure from the relative ground motions. U.S. Pat. No. 3,730,463 issued on Apr. 20, 1971 to Richard, describes a shock mounting apparatus to isolate the building footings. U.S. Pat. No. 4,166,344 issued on Mar. 31, 1977 to Ikonomen describes a system that allows the relative motion of a building structure relative to the ground using frangible links.
Architectural precast concrete can also be designed to mitigate the air pressure effects of a bomb blast. Rigid facades, such as precast concrete, provide needed strength to the building through in-plane shear strength and arching action. However, these potential sources of strength are not usually taken into consideration in conventional design as design requirements do not need those strength measures. Panels are designed for dynamic blast loading rather than the static loading that is more typical. Precast walls, being relatively thin flexural elements, should be designed for a ductile response. There are design tradeoffs between panel stiffness and the load on panel connections. For a surface blast, the most directly affected building elements are the facade and structural members on the lower four stories. Although the walls can be designed to protect the occupants, a very large vehicle bomb at small standoffs will likely breach any reasonably sized wall at the lower levels. There is also a decrease in pressure with height due to the increase in distance and angle of incidence of the air blast. Chunks of concrete dislodged by blast forces move at high speeds and are capable of causing injuries.
Therefore, what is desired is an improved system for connecting pre-cast architectural panels to the structure of the building to accommodate structural movements during earthquakes or high forces due to air pressure events.